1. Field of the Invention
The invention relates to a multilayer polypropylene film which comprises at least one base layer containing polypropylene and migrating additives or a mixture of migrating additives, and at least one outer layer which comprises a mixture or a blend of two polymer components.
The outer layer according to the invention is distinguished by a characteristic matte surface or appearance and is suitable for use as a transfer metallization film.
The invention also relates to a process for the production of the multilayer film and to the use of the film.
2. Description of Related Art
The packaging industry has a high demand for metallized papers and boards. The metal layer ensures good barrier properties against permeation by aroma, grease, moisture and atmospheric oxygen and ensures a highly attractive appearance. Papers having a metal layer are thus highly suitable as a packaging material for tobacco articles, ice-cream, confectionery, candy, chewing gum or fat-containing foodstuffs such as butter or cheese. Other areas of application are cosmetic products or the packaging of sterile articles in the medical sector. A further area of application is the labelling sector. In these cases, the metal layer is usually printed.
For the production of paper/metal composite materials, the transfer metallization process has proven itself in recent years. In this, the metal layer is transferred onto the paper from a polypropylene film which has been vapor-deposited with metal in vacuo. In detail, this process comprises the following individual steps: continuous metallization of the film web by vapor-deposition of the metal layer; coating of the metal layer on the film or of the paper to be metallized with an adhesive; lamination of metallized film and paper to give a film/metal/adhesive/paper composite; curing of the adhesive; and delamination of the composite, i.e., separation at the metal/film interface by peeling off the film. The film used as metal-transfer material is subsequently wound up and is thus available for a remetallization. The use of suitable formulations allows the film to be metallized more than 40 times.
Transfer metallization gives paper having a very smooth, reflective and high-sheen metallic surface. Due to this decorative property, it is frequently used for high-quality gift-wrapping paper, greetings cards and packaging in which the metal layer is printed.
By contrast, other areas of application require that the metal layer of the paper/metal composite have a matte or structured surface or surface areas having different levels of sheen.
German Patent Application P 41 35 096.0 discloses multilayer polyolefin films comprising a polypropylene base layer and an outer layer made from an HDPE blend. This HDPE blend comprises HDPE and copolymers and/or terpolymers made from .alpha.-olefins and, if desired, polypropylene. The outer layer of the multilayer film has minimum sheen and maximum haze, giving the film a characteristic matte appearance.
U.S. Pat. No. 4,419,411 discloses three-layered biaxially oriented polyolefin film which is heat-sealed on both sides. In these films, polysiloxane and silicon dioxide are incorporated into both heat-sealable layers as an additive combination. The base layer essentially comprises polypropylene and contains a small amount of monocarboxamide, some of which migrates from the base layer into the two heat-sealable layers. The multilayer polyolefin film described is said to have a particularly low coefficient of friction.
U.S. Pat. No. 4,578,316 discloses a biaxially oriented multilayer polyolefin film whose base layer comprises a polypropylene homopolymer and whose outer layer comprises a blend of polypropylene and MDPE and/or HDPE. This polyolefin film has a low coefficient of friction together with a readily wettable surface. However, this film also has excellent optical transparency.
WO 89/10839 describes multilayer polyolefin films whose base layer comprises a propylene polymer and whose outer layers comprise either HDPE or an ethylene-propylene copolymer or a polypropylene, the film is stretched under particular temperature conditions. It has, in particular, a low coefficient of friction and good wettability, but simultaneously also high sheen and low haze.
EP-A-0 367 613 discloses a multilayer film comprising a vacuole-containing polypropylene base layer and an inscribable outer layer which contains a first polymer having a melt flow index of .ltoreq.1 g/10 min and a second polymer which is incompatible with said first polymer. Incompatible polymer meaning that the polymers are present as separable phases. An example given of the first polymer is an HDPE having a density from 0.92 to 0.97 g/cm.sup.3. The incompatible polymer is polypropylene or a copolymer of propylene or a terpolymer of propylene. The film described has an opaque appearance, i.e., it is essentially non-transparent to light. The film described has a density of 0.69 g/cm.sup.3. This film cannot be employed for many applications due to its opacity,
EP-A-0 352 463 describes a biaxially stretch-oriented film comprising a base layer with outer layers on both sides, and inert additives, in particular inorganic additives. The three layers are made from polyolefin, and the base layer contains the additives which migrate from the base layer into the outer layers. The film is suitable for the transfer metallization process. The base layer contains the inert additive in the form of finely divided particles. These particles partly project out of the base layer and are fully covered by the outer layers. At least one of the two outer surfaces of the outer layers has increased roughness at least in certain areas as a consequence of the particles projecting out of the base layer. The film is distinguished by a high reuse rate and improved printability of the transferred metal layer. It is also disclosed that the film offers the possibility of providing the transferred metal layer with reduced sheen.
EP-A-0 175 259 describes a biaxially stretch-oriented transfer metallization film based on polypropylene. The film is a three-layer film produced by coextrusion, comprising a base layer of propylene homopolymer and outer layers applied to both sides thereof. The base layer contains an additive supply of migrating monomers in an amount from 0.01 to 2.0% by weight, based on the weight of the polymer forming the base layer. The outer layers comprise polypropylene of lower viscosity than the polypropylene of the base layer.
All known transfer metallization films are unsatisfactory with respect to the appearance of the transferred metal layer. According to the known art, the metal layers have moderate to high sheen, which is disadvantageous for certain applications. The uniformity of the transferred metal layer is frequently unsatisfactory, in particular flaws frequently appear in the form of so-cajoled lightening marks. The lightening marks are caused by poor antistatic properties which causes considerable charging to occur on the film surface, which can in turn result in sudden discharges. The discharges cause increased surface tension at individual points of the film surface, in the form of the lightening marks. These lightening marks on the film are invisible even to the naked eye. However, lightening marks alter the metal adhesion at these points, which results in the lightening marks being visible in the transferred metal layer.
In addition, the known films are unsatisfactory with respect to their make-up properties. Transfer metallization films are used up to 40 times, i.e., are also wound up 40 times. During this, no winding faults may occur, particularly no telescoping of the rolls and no elongation. These problems occur particularly frequently in the case of wide rolls, i.e., those rolls having a width of greater than 1.5 m.